Various damage index (DI) algorithms of detecting changes such as a loosen bolt and a
delamination development in a composite structure were examined using ultrasonic Lamb waves
generated by embedded piezoelectric active sensors. The DI is a single value that is a function of
response signal’s attenuation due to any damage or changes in a structure. Various DI algorithms
such as active damage interrogation (ADI), time domain root men square (RMS), short time Fourier
Transform (STFT) and time reversal (TR) were discussed. For experimental validation, a composite
stiffened panel was used, and loosen bolt damage and low-velocity- impact damage were tested. In
order to pitch and catch Lamb waves, surface mounted PZTs (lead zirconate titanate) were used.
According to the DI algorithms, appropriate ultrasonic guided Lamb waves were selected for
actuators. Each set of DI algorithm and drive signal showed different characteristics to detect the
damage.